Once the plug has been removed, you should inspect it to determine its condition. The condition of a spark plug can tell you a lot about how an engine is operating. In fact, most out-door power equipment technicians will remove the spark plug first when troubleshooting a faulty engine.
One of the first things to check is whether the spark plug is the correct type for the engine. Then, check the condition of the electrodes. Figure 46 displays several different engine con-ditions that are revealed by spark plugs. Figure 46A shows a new spark plug. Note that the bottom surface of the center electrode is flat and the surfaces of the lower electrode are squared. A used plug in normal condition will look much the same, but the electrodes will be colored an ashy gray or light tan from carbon deposits. (Carbon deposits are produced during normal fuel combustion.)
An oil-fouled plug is shown in Figure 46B. Oil fouling will cause the end of a plug to be saturated with wet, sooty, black oil deposits. In a four-stroke engine, an oil-fouled plug may indicate that the piston rings aren’t sealing the
cylin-area. Sometimes, a clogged breather can cause an oil-fouled plug. (Remember that a breather is a vent in the crankcase.) The clogged breather will prevent the crankcase from venting properly, and as a result, pressure will build up in the crank-case. This pressure will cause oil to be pushed up past the piston rings and into the combustion chamber. The oil in the combustion chamber will then foul the spark plug.
On two-stroke engines, oil fouling of spark plugs is quite com-mon. Remember that in a two-stroke engine, the oil and the fuel are mixed together in the crankcase. Therefore, oil fouling is a normal by-product of engine operation in the two-stroke engine. Oil fouling in a two-stroke engine plug may also be caused by too much oil in the fuel-and-oil mixture. For exam-ple, if an engine is designed for a 40;1 fuel-and-oil mix and your customer is using a 10;1 mixture, the plug can easily
FIGURE 46—Shown here are three of the differ-ent conditions that are possible when a spark plug is removed from an engine. Figure 46A shows a new, clean plug. Figure 46B shows an oil-fouled plug.
Figure 46C shows a fuel-fouled plug.
become oil fouled. (Note that in either a two-stroke or four-stroke engine, oil fouling may also be displayed at the exhaust pipe as excessive smoke.)
Figure 46C displays a spark plug that was fouled by exces-sive fuel. Fuel fouling (also called carbon fouling) is indicated by dry, black, fluffy deposits on the spark plug electrodes.
However, the plug won’t have the caked or lumpy appearance of an oil-fouled spark plug.
Fuel fouling is most often caused by extended operation with an air-and-fuel mixture that’s too rich. This is usually a car-buretor problem, although a blocked exhaust or faulty valve can also cause fuel fouling. You’ll probably be able to smell fuel on the spark plug if the fuel fouling problem is severe.
Another possible cause of fuel fouling is weak ignition. If the high-tension cable, points, condenser, electronic module, or coil is faulty and the spark is too weak, a plug can become fuel fouled. Fuel fouling can also be caused by using too cold a spark plug in an engine.
Both oil fouling and fuel fouling can cause a spark plug con-dition known as a bridged gap. In this situation, carbon or oil deposits build up in the spark plug gap until it becomes com-pletely blocked. A bridged gap will seriously affect the engine’s ignition efficiency.
Note that the deposits caused by fuel and oil fouling can usu-ally be cleaned off a spark plug, and the plug can then be reinstalled in the cylinder head. However, this isn’t usually a cost-effective practice. Spark plugs are inexpensive, and they should always be replaced during an engine tune-up.
After many hours of use, spark plug electrodes will begin to erode. New electrodes have flat surfaces; however, an eroded center electrode will appear rounded, while an eroded side electrode will have a curve on its inside surface. Plugs with eroded electrodes should be replaced.
When inspecting a spark plug, you may find that the plug’s electrode or insulator is damaged. The electrodes may be heavily pitted and the insulator broken or cracked in extreme situations. This damage is usually caused by too hot a plug being used in an engine. A physical impact can also damage
the spark plug, you may find damaged or bent electrodes or cracked and broken insulators. Or, if the spark plug reach is too long, the piston head may strike the electrodes. The most common cause of physical damage, however, is debris or for-eign objects in the cylinder. Sometimes, a bolt or washer may loosen and actually be “sucked in” to the cylinder. The foreign object will then strike the spark plug electrodes when the pis-ton rises.
Plug heat ranges are changed depending on the condition of the plug that’s removed from the cylinder head. A hotter plug is generally installed if the plug looks dirty. A cooler plug is installed if the plug displays heat damage such as the cracking or chipping of the insulator. The manufacturer’s manual will provide recommendations about the type of plug that should be used in their engine. You should always follow these recom-mendations to prevent the types of problems we described.
Never sand, sandblast, or file a spark plug and then replace it in an engine. Using sandpaper or a file will leave tiny grooves on the electrodes. These grooves will either burn off or will collect deposits as the engine operates. Also, sandblasting and filing will leave tiny particles of sand or metal behind on the electrodes. These particles will get into the engine’s cylinder and cause serious damage.
In the past, some spark plug manufacturers have produced small sandblasting cleaning machines that were designed to be used with their spark plugs. However, most small engine manufacturers now recommend against using these machines for the reasons we described. Remember, spark plugs are inexpensive. If you’re ever in doubt of a plug’s quality, simply replace it.